From DT Online
The total brightness of a lamp is measured in the SI base unit of Candelas but for practical design purposes, it is more important to measure the amount of visible light available in a particular direction and this is measured in Lumens.
The amount of light actually reaching a particular surface can vary according to the type of lamps used, their total wattage, their distance from the surface, and whether or not they are modified by a shade or diffuser for example. This is known as Illuminance and is measured in lumens per square metre or Lux.
- a 100 watt filament bulb or its equivalent may generate 1200 lumens (from tables or manufacturer's information)
- it is possible that, because this lamp is shaded and positioned on a high ceiling, only 15% of its light reaches a work-surface below;
- the actual light reaching the work-surface is therefore only 15% of 1200 which is 180 Lumens;
- assuming a standard length of kitchen worktop 3 metres long x 600mm wide (i.e. area = 1.8 sq.m.);
- Illuminance (i.e. light arriving at the work-surface divided by its area) is 180 ÷ 1.8 = 100 Lumens per square metre or Lux.